A microporous membrane has various porous diameters, porous shaped, and the porous numbers, and has been utilized in wide variety of fields due to characteristics capable of exhibiting by its specific structure. For example, a microporous membrane is used as, for example, a separating membrane used for a water treatment, concentration, etc, utilizing a sieve effect due to difference in a porous diameter, an adsorption sheet used in materials for water absorption, oil absorption, and deodorant, which utilize a large surface area and a porous space with microporousness, a moisture permeable waterproof sheet utilizing a feature that air and water vapor are permeated but water is not permeated due to difference in a molecular size, a polymer electrolyte membrane and a moisturizing membrane useful for a fuel battery, and the like, which have many functions by filling various materials in porous spaces, and further, a liquid crystal material, and a battery material.
In recent years, from the viewpoints of saving energy and resources on the background of enhancing international global environment protection activities, particularly in the automobile industry, an electrical automobile (PEV) and a hybrid automobile (HEV) have been positively introduced and studied, and as motor driving power and auxiliary power thereof, a fuel battery and a large lithium ion secondary battery have been actively developed. Further, an electrical double layer capacitor capable of charge and discharge of large current immediately has been expected as an auxiliary power for HEV and its development has been promoted. In a lithium ion secondary battery and an accumulator battery such as an electrical double layer capacitor, a porous membrane keeping an electrolyte therein, called a separator having functions of preventing contact between a cathode and an anode and allowing ion to permeate is provided between the cathode and the anode. It is intensively required for a separator to have high safety along with having a higher energy density of an accumulator battery, and long-term reliability along with development for automobile application, and the like.
However, a shape of an active material of an electrode is not necessarily flat and smooth, and there is a possibility that the active material protrudes through a separator to short-circuit between electrodes, and from the viewpoint of prevention of short-circuit, high puncture strength is desired. Further, when a foreign object such as a fallen active material exists, the electrodes generate swelling and shrinkage by repeating charge and discharge of a battery, and there is a possibility that the foreign object is pushed to the separator for long period of time to thereby penetrate the separator and reach short-circuit. Further, along with having a high energy density and high output, a rolled-up structure and a collector structure are diversified and pressure distribution in a battery is not uniform, and a high pressure may be continuously applied locally on a separator in some cases, and in such cases, there is a possibility to reach short-circuit.
In order to solve these problems of safety and long-term reliability, various studies have been made, but these have not been necessarily satisfactory.
For example, Patent Document 1 discloses a microporous membrane made of polyethylene and polypropylene having high puncture strength and excellent in compression characteristics. However, in the disclosed polyolefin microporous membrane, an effect is exhibited under pressurization for such a short time as 60 seconds, but a concern for the compression characteristics under pressurization for a long time is remained because of being solely polyolefin.
In Patent Document 2, improvement in compression characteristics is proposed by finely dispersing a non-polyethylene thermoplastic resin in a polyethylene microporous membrane. However, in Patent Document 2, since small pores made by craze-form gaps is formed by cleave with the center on the non-polyethylene thermoplastic resin dispersed in polyethylene, a fibril itself forming a porous structure is not reinforced, and thus, a concern on the compression characteristics in a local minute area is remained.
Patent Document 3 discloses a porous membrane made of fillers such as calcium carbonate and barium sulfate, high-density polyethylene, and a low molecular weight compound and stretched longitudinally and laterally and traversely each by 3 times or more. However, the process of Patent Document 3 is a pore developing technique by boundary separation of polyethylene and fillers, a fibril itself forming a porous structure is not reinforced, and thus, a concern on the compression characteristics in a local minute area is remained. Further, since boundary separation is performed by stretching in preference to extension of ramera and rearrangement in the production method, there is a limitation on high puncture strength. Furthermore, since there is a possibility to accidentally generate a void with a large porous diameter in the boundary separation, when using as a separator of a lithium ion battery etc., there is a concern on reliability such as slight short-circuit and self discharge.
Patent Document 4 discloses a porous membrane made of a polyolefin resin containing 20 to 80% by weight of inorganic particles such as anhydrous silicic acid and titanium oxide. However, a draw ratio is as low as about 6 times, and high puncture strength can not be achieved. Further, in the case of stretching by a large ratio, a porous structure is coarsened with a starting point of an agglomerate, and further, membrane rupture is likely to be caused. Patent Document 5 proposes a porous film made of an ultra-high molecular weight polyolefin resin having a weight average molecular weight of 500,000 or more and 5 to 70% by weight of particles having a particle diameter of 0.001 to 10 μm. However, a porous film disclosed in Examples has a draw ratio is as small as 2×2 times, and thus high puncture strength can not be achieved. In addition, no description regarding long-term compression characteristics is shown in Patent documents 4 and 5.
Patent document 6 proposes a separator in which an inorganic material such as alumina, silica and zirconia having an average particle diameter of 5 to 100 nm is present on and in a flexible substrate such as a nonwoven fabric. However, since the substrate is a flexible nonwoven fabric, or the like, there is a limitation on high puncture strength and compression resistance characteristics. Further, inorganic particles are filled in a void part or a surface layer of a nonwoven fabric etc. in a posterior step, and thus a concern such as falling down of inorganic particles is remained.
In Patent Document 7, a polyolefin microporous membrane containing an inorganic particle having high puncture strength is disclosed, but there is no description regarding a particle diameter and a minute area of the inorganic particle and compression resistance under giving a load for a long term.    Patent Document 1: Japanese Patent Application Laid-Open No. 2002-194132    Patent Document 2: Japanese Patent Application Laid-Open No. 2004-161899    Patent Document 3: Japanese Patent Application Laid-Open No. 2003-82139    Patent Document 4: Japanese Patent Application Laid-Open No. 2000-208123    Patent Document 5: Japanese Patent Application Laid-Open No. 2003-26847    Patent Document 6: National Publication of International Patent Application No. 2005-536857    Patent Document 7: WO No. 2006-25323